The formation of conventional light emitting diodes (LEDs) made entirely from inorganic semiconductor materials such as GaAs and GaN has been extensively described in the literature such as in Chapter 12 of the Handbook of Optics, Vol. 1, McGraw Hill, Inc. (1995) ("Light Emitting Diodes" by R. H. Haitz et al.) and by S. Nakamura et al. in Appl. Phys. Lett. 64, 1687 (1994). Light emitting diodes made entirely from organic materials have also been described in the literature such as by C. W. Tang et al. in J. Appl. Phys. 65, 3610 (1989) and by C. W. Tang in Society for Information Display (SID) Digest, 181 (1996).
Semiconductor LEDs are widely used today. However, while robust blue, green, and red light emitting diodes are available, separate structures, materials, and growth processes are required to achieve the different colors thus resulting in completely different devices. Blue and green light emitting diodes are made of alloys of InGaAlN with each color requiring a uniquely different alloy composition. Red LEDs are made of InGaAsP which is an entirely different compound altogether.
Fully organic LEDs (OLEDs) offer the advantage that color can be changed from blue to red/orange by simply adding dyes in minute amounts to the optically active organic electroluminescent layer. Alternatively, color conversion can be achieved by coating the OLED with organic materials which act to convert the light emitted to longer wavelengths (color converters). However, limitations of OLEDs as the underlying light source include problems with degradation of diode performance during electrical operation and the unavailability of efficient blue emitting materials. An additional problem is that of the sensitivity of organic OLEDs to subsequent processing in that they cannot be subjected to temperatures above, typically, 100.degree. C. and cannot be exposed to solvents such as water. These limitations call for more robust OLEDs.